The COVID-19 pandemic and the consequent restrictions imposed by governments worldwide have had profound social and psychological effects, particularly for young adults. This study used longitudinal data to characterise effects on mental health and behaviour in a UK student sample, measuring sleep quality and diurnal preference, depression and anxiety symptoms, wellbeing and loneliness, and alcohol use. Self-report data was collected from 254 undergraduates (219 females) at a UK university at two-time points: autumn 2019 (baseline, pre-pandemic) and April/May 2020 (under 'lockdown' conditions). Longitudinal analyses showed a significant rise in depression symptoms and a reduction in wellbeing at lockdown. Over a third of the sample could be classed as clinically depressed at lockdown compared to 15% at baseline. Sleep quality was not affected across the sample as a whole. The increase in depression symptoms was highly correlated with worsened sleep quality. A reduction in alcohol use, and a significant shift towards an 'evening' diurnal preference, were also observed. Levels of worry surrounding contracting COVID-19 were high. Results highlight the urgent need for strategies to support young people's mental health: alleviating worries around contracting COVID, and supporting good sleep quality, could benefit young adults' mental health as the COVID-19 crisis unfolds.